Apparently, seed color in wheat and oat is produced by one, two or three genes. When two or three genes for color are present together, they seem to interact in the duplicate manner so that white seed color is produced only when, all the genes are present in the recessive state.
However, on a close examination of the colored seeds; Nilsson-Ehle found that there were marked differences in the intensity of their color. Therefore, he further classified the colored seeds on the basis of intensity of their red color.
This hypothesis assumes that a given quantitative character is controlled by a series of independent genes having additive and cumulative effects.
Each gene is assumed to contribute a unit of quantity e.g. height, weight, kernel color in wheat. In wheat, two identical genes produce Red color: R, and Rr
When red kerneled variety crossed with white kerneled one, F, had intermediate red color i.e. less red than homozygous red parent while in F, several shades were observed i.e. dark red to white in the proportion of 1 : 4 : 6 : 4 : 1.
Hypothetical model for quantitative inheritance:
Following kind of model experiment can be used to study quantitative characters like plant height.
1. It is presumed that environmental factors are controlled and uniform.
2. True breeding parents are involved in crossing.
3. The height is controlled by three pairs of factors.
4. Each gene contributes 3? height.